Rivets are a preferred fastener for use in assembling aluminum and titanium airframe components because they provide a joint having a high shear strength. This strength arises from the interference fit between a rivet and the components it joins. Commercial aircraft incorporate millions of riveted joints. Riveting is the largest single cost operation associated with airframe manufacturing. In addition, rivets are also the fastener of choice for use in producing automobile and truck bodies, mobile homes, and light rail vehicles.
The conventional impact riveting process is time consuming and causes repetitive stress health problems related to operator handling of impact tools. This process often requires two operators, positioned on opposite sides of the work piece, further increasing costs and limiting application of this process for joining components in structures having limited access. Hydraulic squeeze riveting technology avoids the repetitive stress injury associated with impact riveting, but this process typically employs large hydraulic tools, which are not readily adapted to join components in complex structures or to meet constantly changing productions needs. Achieving a reliable interference fit while riveting components requires match-drilled holes made with precise dimensional control.
A riveting process that does not require operators on both sides of the pieces being joined is referred to as blind hole fastening. Blind hole fastening has the attraction of potential tremendous labor cost savings. Existing blind hole fasteners are typically variations of some type of pull-through swaging process or use threaded expandable fasteners. These systems are all relatively complex, involving tapered sleeve mechanisms. The structural performance of multi-component fasteners of the type often used for blind hole joints is inherently inferior to a single piece fastener. There are currently no structural, blind-hole fasteners in widespread use for structural airframe manufacturing.
It would be desirable to develop a simple method for riveting, which uses compact, easy to manipulate tools, do not use multi-component fasteners, do not subject the operator to significant vibrational force, and which can be used for setting blind hole fasteners. Currently, such methods are not provided in the prior art.
While rivets are currently the preferred type of fastener for assembling aluminum and titanium airframe components, riveting is labor intensive and costly. Such a labor intensive process is poorly suited to high volume manufacturing applications. Aluminum welding is appropriate for some components, but this process requires skilled workers, careful quality control, and post processing to relieve stresses in the welded assembly. Explosive bonding of metal structures has been investigated as a means of directly bonding aluminum alloys. This process involves the detonation of high explosive sheets against plates of materials to be bonded. The high velocity impact of the two plates generates extremely high stresses at the material interface, allowing the materials to diffusion bond and interlock mechanically. This process is not suited for high volume continuous manufacturing of structures such as airframes and vehicles.
It would be desirable to develop an economical alternative to conventional riveting and the use of hydraulic riveting tools for joining two components that does not require the skill, or careful attention to quality control, and the post processing required for aluminum welding, and which is also well suited for high volume continuous manufacturing. Currently, such alternatives do not appear evident in the prior art.
Another prior art process for joining two components, which can be employed as an alternative to riveting, is to provide an interlocking clinch fastener between two components, such as two sheets of metal. Mechanical clinch forming has become accepted practice as an alternative to tack welding in a number of manufacturing applications, including the production of appliances, automotive, and general sheet metal fabrication (e.g., in the production of duct work and electrical fixtures). Mechanical clinch fastening systems that use a punch and die are well known and in common use.
Disadvantages of these prior art clinch forming methods are that they are limited to only being usable with highly ductile, relatively thin sheet steel and some aluminum alloys. Thicker steel alloys, structural aluminum, and titanium cannot be joined using existing clinch forming methods. Furthermore, the stiff, precisely aligned punch and die combination is not suited for large structures. The associated tooling generally comprises a very large, heavy, and expensive structure. The process cycle time is relatively slow, and only one size clinch may be formed by a specific tool. The process is limited to forming a circular clinch, which may not always be the desired shape. Finally, punch wear and breakage is a problem with mechanical clinch forming.
It would be desirable to provide a clinch forming method that does not suffer from the above-noted disadvantages. The method should preferably be suited to high volume and flexible manufacturing of large aluminum structures, such as appliances, ductwork, automobiles, trucks, recreational vehicles, and trailer homes; and should provide stronger joints than existing prior art techniques.
Mechanical cold-working of holes is currently used by aircraft manufacturers to improve the fatigue resistance of critical structures that are joined with rivets or other fasteners. Mechanical cold working of holes involves applying a radial force to the inside surface of the hole. The residual compressive stress resulting from plastic radial strain around cold-worked holes is known to result in an improved fatigue life for the hole. The conventional process of mechanical cold working of rivet holes includes the following steps. After the holes are drilled, a special gauge is inserted to verify hole dimensions. A disposable sleeve is placed in the hole and a hydraulically-actuated mandrel is drawn through the hole. The hydraulic pressure causes the mandrel diameter to expand while the mandrel is in the hole. A second inspection is performed to verify that the diameter is still within tolerance and that the proper expansion and desired cold working has occurred. It will be apparent that these steps comprise a time consuming, labor-intensive process.
It would be desirable to provide a method for the mechanical cold working of holes that is much less labor intensive. This new method should preferably be capable of forming and cold working holes in aluminum in a single step.
Over the past 20 years, surface impact treatment has emerged as an important technique to enhance the fatigue life of metal components by introducing a residual compressive stress in a thin surface layer. In particular, shot peening can significantly enhance the fatigue life of welded structures by eliminating the residual tensile stress that occurs due to thermal contraction at the weld toe. Shot peening is often used for surface impact treatment (as opposed to cleaning) and is carried out by directing a stream of small (typically under 1 mm) steel shot at a metal surface. One inherent disadvantage of this process is that it must be carried out in a confined space, because the shot ricochet wildly from the work piece in many directions. In addition, the process generates considerable metal dust and noise. As a result, shot peening is carried out as a batch process on assembled structures. In order to ensure a good surface finish, the shot must be clean, round, and free from broken material. Shot peening at the high velocities required for surface cold working results in breakage of a significant fraction of the shot; the broken shot must be removed and replaced at significant cost.
It would thus be desirable to develop a method for introducing a residual compressive stress in a thin surface layer of a piece, which can be carried out as a continual process, rather than a batch process, and which does not include the disadvantages of ricocheting or broken shot. Such a method can be integrated with robotic welding to provide weld stress relief and cold work following the welding operation. The prior art does not disclose such a method.